FREE-LTVTNG NEMATODES 475 



four female cells is potential, the other three being the so-called 

 polar bodies which are left at the periphery of the egg to disinte- 

 grate and disappear. The polar bodies are to be looked for in 

 eggs that have just entered the uterus, and can be observed to 

 advantage only in stained specimens, though they may sometimes 

 be seen in the living material. The fundamental facts connected 

 with fertilization and inheritance in animals were first worked out 

 largely through the instrumentality of the eggs of various species 

 of nematodes. In this respect they are classical objects. 



FIG. 771. Forms of spicula. i. Broad, tapering, blunt. 2. Elongate. 3. Slender. 4. Setaceous. 

 5. Elongated, tapering. 6. Elongated, arcuate. 7. Elongated, bent. 8. Fusiform, slightly arcuate. 

 9. Arcuate, strongly cephalated. 10. Sickle-form, n. Hamate. 12. L-shaped. (After Cobb.) 



The male intromittent organs, the spicula, are usually two in 

 number, and in nearly all free-living species the two are identical 

 in form and size. Each spiculum is usually a straight, curved, or 

 bent, elongated framework of cuticula, commonly one to two times 

 as long as the anal body diameter. Exceptionally it may be very 

 long and slender. The main portion of its shaft is usually of uni- 

 form size, while the free or distal end commonly terminates in a 

 somewhat blunt point, which, however, may be variously modified. 

 The anterior or proximal end is often swollen or cephalated, for the 

 attachment of muscles. 



The muscle for protruding the spiculum more or less insheaths 

 it, and is attached to the proximal end of the spiculum and to the 

 body wall, or to an accessory piece, near the anus, so that its con- 

 traction moves the spiculum toward the anus and thus protrudes 

 it. The retractor muscle is attached to the proximal end of the 

 spiculum and thence usually passes forward and toward the dorsal 

 side of the body, where it is attached to the body wall; its con- 

 traction thus tends to pull the spiculum back into the body. It is 

 usually rather easy to observe these retractor muscles of the spicula, 

 but difficult to observe the protruding muscles. 



In order that these muscles may act to better advantage the 

 spicula often slide in grooved pieces of cuticula named the acces- 



